The present invention generally relates to imaging system calibration. In particular, the present invention relates to calibration of pixel spacing based on the patient in an X-ray mammography system.
Medical diagnostic imaging systems encompass a variety of imaging modalities, such as X-ray systems, computerized tomography (CT) systems, ultrasound systems, electron beam tomography (EBT) systems, magnetic resonance (MR) systems, and the like. Medical diagnostic imaging systems generate images of an object, such as a patient, for example, through exposure to an energy source, such as X-rays passing through a patient, for example. The generated images may be used for many purposes.
In order to help ensure that medical diagnostic images are reliable, it is advantageous to calibrate medical diagnostic imaging systems. The calibration of imaging systems is important for several reasons, including image quality, measurement accuracy and system performance. Inaccuracies in an imaging system may result in blurring, streaking, or introduction of ghost images or artifacts in the resulting image. Poor image calibration may prevent reliable analysis of an image. The calibration of medical imaging systems may help to produce a distinct and usable representation of an object.
The Digital Imaging and Communication in Medicine (DICOM) standard provides rules and guidelines for medical image acquisition, transmission, storage, and retrieval. These rules, guidelines, and other limitations impact how imaging systems and other related systems, such as reviewing and/or processing workstations, acquire and process images. For example, DICOM specifies a Digital X-ray (DX) Detector Distance Source to Patient Type 3 Implementation limitation. In a magnification mode, the value is equal to a distance in millimeters from the source to a theoretical average breast height (e.g., 20 mm above the breast support). In a contact mode, the value is equal to a distance in millimeters from the source to the detector plane.
As another example, in DICOM, an Estimated Radiographic Magnification factor Type 3 and a Distance Source to Entrance are computed based on the Distance Source to Patient. A DX Detector Imager Pixel Spacing Type 1 Implementation limitation specifies that the value is equal to a physical distance measured at a theoretical average breast height (e.g., 20 mm above the breast support) between the centers of each projected image pixel.
The Integrating the Healthcare Enterprise (IHE) is an initiative by healthcare professionals to improve sharing of information between healthcare computer systems. IHE coordinates and promotes use of established standards, such as DICOM, to facilitate communication and effective use of information for healthcare purposes. IHE is developing profiles to define requirements for certain system and/or certain behavior. For example, IHE is developing an IHE Mammography Image Profile (IHE Mammo) to define mammography requirements for display on Picture Archiving and Communication Systems (PACS) and third-party workstations. The profile includes recommendations regarding viewing of mammography images including image size, pixel spacing, etc.
Many third-party workstation and PACS vendors have received FDA clearance for their viewing stations to be used for digital mammography, which has introduced a variety of interconnectivity and interoperability issues with digital mammography imaging systems. The IHE Mammo profile helps to address these interconnectivity and interoperability issues for image acquisition and display.
One issue for interoperability is displayed image size, particularly when current and prior images have been obtained on different equipment. Regardless of the physical size of the detector plate, pixels, or gantry geometry, images should be displayed with the same relative physical size to radiologists. The DICOM standard already requires that the necessary attributes be present and completed. The IHE Mammo profile requires that a workstation make use of those attributes. Thus, correctness of those attributes is an important issue that needs to be address, but has not yet been in several circumstances.
An Imager Pixel Spacing (0018,1164) attribute is defined to specify a physical distance measured at a front plane of an image receptor housing between a center of each pixel. Pixel Spacing (0028,0030) is an attribute specifying a physical distance in a patient between a center of each pixel. However, displays often vary in choosing Imager Pixel Spacing (0018,1164) or Pixel Spacing (0028,0030) as the basis for performing measurements, display, and/or printing. Thus, correlation or use of these values needs to be addressed.
In certain mammography X-ray systems, a calibration plane lies at a front plane of the X-ray system detector housing. An image acquired without a zoom bench may include the following parameters:
Detector Active Dimension(s) 250, 286
Distance Source to Detector
Distance Source to Patient
Estimated Radiographic Magnification 1
Imager Pixel Spacing 7.000E-2
Pixel Spacing 7.000E-2.
A image acquired with a zoom bench may include the following parameters:
Distance Source to Detector
Distance Source to Patient
Estimated Radiographic Magnification 1.8
Imager Pixel Spacing 7.000E-2
Pixel Spacing 3.890E-2.
In certain mammography X-ray systems, a calibration plane lies somewhere above a front plane of the X-ray system detector housing. An image acquired without a zoom bench may include the following parameters:
Field of View Dimension(s) 286, 233
Distance Source to Detector 650
Distance Source to Patient 636
Estimated Radiographic Magnification 1.022
Imager Pixel Spacing 7.000E-2.
A image acquired with a zoom bench may include the following parameters:
Distance Source to Detector 650
Distance Source to Patient 392
Estimated Radiographic Magnification 1.6582
Imager Pixel Spacing 7.000E-2.
In certain mammography X-ray systems, a calibration plane lies 20 mm above a front plane of the X-ray system detector housing. An image acquired without a zoom bench may include the following parameters:
Field of View Dimension(s) 306, 239
Distance Source to Detector 660
Distance Source to Patient 660
Estimated Radiographic Magnification 1
Imager Pixel Spacing 0.09409091.
A image acquired with a zoom bench may include the following parameters:
Distance Source to Detector 660
Distance Source to Patient 366
Estimated Radiographic Magnification 1.8
Imager Pixel Spacing 5.545E-2.
The data above illustrates problems with IHE Mammo profile section 4.16.4.2.2.1.1.3, in which Imager Pixel Spacing (0018,1164) and Estimated Radiographic Magnification Factor (0018,1114) are used to estimate the physical size of the image pixels in the patient body. The physical size of the pixels in an image for the purposes of distance measurements and the display of a distance caliper shall be approximated using the values of Imager Pixel Spacing (0018,1164) divided by Estimated Radiographic Magnification Factor (0018,1114). However, implementing that part of IHE Mammo profile will cause a “double correction” to certain mammography images.
Additionally, DICOM CP-586 clarified that Pixel Spacing (0028,0030) is allowed in projection X-ray and DX family images, CP-586 specifies that Pixel Spacing (0028,0030) shall have the same values as Imager Pixel Spacing (0018,1164) if the image is not calibrated or corrected. Otherwise, Pixel Spacing (0028,0030) shall have the corrected or calibrated values. The IHE Mammo profile may reintroduce double correction to other DX images which had been resolved by DICOM CP-586.
Further ambiguity issues are also introduced with the Estimated Radiographic Magnification Factor (0018,1114) for images acquired with a zoom bench versus images acquired without a zoom bench.